Semiconductor integrated circuits are the fundamental building blocks of modern electronic devices. Computers, cellular phones, and consumer electronics rely extensively on these devices, which may be used for storage of, computations on, and communication of data.
The most common semiconductor devices are formed using silicon as the primary substrate substance. Layers and regions of N-type material, P-type material, and semi-insulating material are combined to form electronic devices and circuits. N-type material is material in which excess electrons act as charge carriers. In a P-type material, holes (missing electrons) act as charge carriers for the flow of electricity. A semi-insulator material is one which has a high resistance to current flow and may be used to isolate components of a circuit or a device, and act as a substrate on which active devices may be epitaxially grown. Shallow level impurity dopants are generally expected to provide conductive qualities to produce N-type and P-type materials, while deep level impurity dopants provide resistance to current flow by acting as traps for any charge carriers overcome only by significant ionization energy to thereby produce semi-insulating material.
The arrangement of P-type, N-type, and insulative materials and the respective electrical connections to each will determine what type of electrical device is created. Transistors, diodes, capacitors and most other electrical devices are created through the arrangement of these materials in a semiconductor device.
Recently, the advantages of using the Group III-V semiconductors (semiconductors formed from compound alloys including Group III and Group V elements) instead of silicon have led to extensive research and development. Among the typically used compounds and alloys are gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs) and indium gallium phosphide (InGaP). The basic designs for the transistors and other devices used in silicon-based electronic devices have been adapted to Group III-V materials. Devices made from the Group III-V materials generally require lower power and are faster (operate at high frequencies).
Group III-V semiconductor material may also be used to produce optoelectronic devices, such as semiconductor lasers. In such devices an active region of undoped or low-shallow level doped semiconductor material that is sandwiched between dual layers of P-type and N-type shallow level doped materials emits coherent light in response to the application of electrical current. The light is produced when holes from the P-type material recombine with electrons from the N-type material in the active region.
Other applications of the Group III-V materials are known to those in the art and include optical detectors, high speed amplifiers and logic circuits. The widespread substitution of these semiconductors for silicon devices is impeded by relative difficulty and expense in producing Group III-V semiconductors and semi-insulators in comparison to the silicon devices.
One difficulty in producing Group III-V devices concerns the processes used for the production of GaAs compatible semi-insulating layers. GaAs is the primary building block for typical Group III-V devices. A preferred technique for production of commercial GaAs is the LP-MOCVD process, since it is well suited to mass production.
Several semi-insulating GaAs-based layers have been produced by the LP-MOCVD technique. However, these techniques typically require the use of an extrinsic deep level dopant during growth to produce highly resistive material. Generally, two types of deep level impurity dopants have been used: transition metals, such as iron, and oxygen.
Transition metals are problematic because of their high diffusivity in semiconductor material such as GaAs. Thus, the dopant will diffuse out of the highly resistive layer during the growth of subsequent layers or thermal cycling during device fabrication, contaminating neighboring epitaxial layers.
One particular transition metal doping technique is iron doping. Typically, a precursor of iron pentacarbonyl or ferrocene is used in conjunction with MOCVD growth of epitaxial phosphorous containing layers. High resistivity on the order of 10.sup.9 ohm-cm is realized through this technique.
Such iron doping techniques have a number of difficulties. One of the difficulties is recognized by Dentai et al., U.S. Pat. No. 4,782,034. That patent noted that iron doped indium phosphide layers have poor thermal stability, i.e., performance is sensitive to temperature. Addressing this problem, the Dentai patent adopts doping using a titanium-based metal-organic dopant precursor. Similar to iron doping techniques, fairly high temperature is used in the growth to decompose the precursor according to Dentai, on the order of 650.degree. C. Dentai contemplates decomposition of the titanium precursors at temperatures of up to 850.degree. C. Temperatures on this order may induce dopant diffusion which reduces the degree of control over the location of growth of the insulating material, thereby leading to the contamination of neighboring layers.
Further difficulties may arise from the nature of the precursors used for iron doping and other transition metal doping techniques. The aforementioned ferrocene and iron pentacarbonyl tend to leave behind a residue in the reactor. The residue then may act as a contaminant during further growth in the reactor. Thus, a separate crystal growth chamber system is sometimes dedicated to the growth of the iron-doped indium phosphide. This is expensive since a commercial growth reactor may cost one million dollars or more. Oxygen doping to produce high-resistivity GaAs-based material has similar drawbacks. Particularly, the oxygen dopant source can contaminate the reactor chamber so that subsequently grown layers will also be deep level doped with oxygen, which is undesirable. Both of these techniques therefore make it difficult to integrate device quality epitaxy and highly resistive layers into the same growth run or using the same growth chamber. Similar problems are expected for In.sub.0.49 Ga.sub.0.51 P doped with transition metals or oxygen.
Another extrinsic technique, preferable to the transition metal techniques, is the halide doping technique of Gardner et al., commonly assigned U.S. patent application Ser. No. 08/410,782, filed Mar. 24, 1995. That technique produced good resistivites on the order of 10.sup.9 ohm-cm through LP-MOCVD growth without post-processing annealing. While this is an efficient process, some complexity is added by the need for a dopant source. In addition, certain ones of the halide dopant sources, such as CC1.sub.4 are highly regulated due to environmental concerns.
A photoassisted MOCVD process has also been proposed to produce semi-insulating materials. See, Roberts et al., "Low-Temperature Growth of High Resistivity GaAs by Photoassisted Metalorganic Chemical Vapor Deposition", Appl. Phys. Lett. 64 (18), May 2, 1994. However, the highest nonannealed resistivity obtained was about 10.sup.6 ohm-cm. This is below typical commercially acceptable semi-insulators, which are on the order of 10.sup.7 ohm-cm. In addition, the photoassisted technique involves the raster scanning of laser light during the reaction. This adds complexity to the growth system, and may not be easily adapted to larger area layers than those produced experimentally since the demonstrated scan length was about 1 mm.
GS-MBE has also been used to produce GaAs compatible semi-insulating layers of phosphorous containing materials. Specifically, high-resistivity In.sub.0.49 Ga.sub.0.51 P (referred to as InGaP) has been demonstrated by GS-MBE. The GS-MBE layers are typically grown at below standard temperature and do not require an extrinsic dopant source to obtain "as-grown" semi-insulating resistivities of up to about 10.sup.6 .OMEGA.-cm. To achieve higher resistivities that are more acceptable for use as isolation regions (&gt;10.sup.6 .OMEGA.-cm, 10.sup.7 -10.sup.9 are typically considered good semi-insulator levels), however, the material must be annealed at 600.degree. C. for 60 minutes. Annealing procedures are not desirable because thermally cycling epitaxial layers can cause dopant diffusion into neighboring layers or intermixing of atoms at interfaces between different materials. The GS-MBE growth technique is also not as commercially desirable as LP-MOCVD.